Microstructural study of V2D by X-ray measurements
Using three different methods of X-ray measurement, vanadium host-atom displacements due to interstitial deuterium atoms were measured at room temperature. Assuming a super-structure long-range order parameter, S = l, at room temperature, we have shown that the displacement along the pseudo-tetragonal c direction from average position in the ordered structure (DAZ) is 0.11 A. Assuming S=0.85 at room temperature, we determined DAZ to be 0.12 A. From a consideration of the distortion tensor DAZ is 0.17 A. Therefore, we conclude that DAZ lies between 0.11 A and 0.17 A. The temperature dependence of S was measured using the superstructure reflection intensity. In addition, the temperature dependence of the a and c parameters of the ordered structure was measured. By using a simple assumption relating S to the site occupancy in the ordered V[lowered 2]D structure, we calculated a and c and compared these values with the measured ones. They fitted well except near the critical temperature T[lowered c] where the calculated tetragonality is smaller than the measured tetragonallty. This indicates that just below the critical temperature the ordered tetragonallty changes slowly while S decreases more rapidly. In other words the calculated tetragonallty, based on random tetrahedral occupancy for the disordered deuterium, falls off more rapidly than the measurements. Diffuse intensity due to short range order (SRO) among deuteriums was also observed above T[lowered c]. Even though the intensity was quite weak we attempted to determine the interstitial occupancy most consistent with this SRO diffuse intensity. Finally, carrying out further calculations similar to those of Khachaturyan, we have obtained temperature-dependent occupation probabilities for each interstitial site.